This case report addresses the diagnosis and treatment of pyometra in a 10 year old Domestic Shorthair queen who presented for evaluation of a suspected space occupying lesion and abdominal effusion. Diagnosis of pyometra was based on signalment, history, clinical signs, physical exam findings, and diagnostic imaging. Pyometra is a uterine infection typically diagnosed in older animals. It is believed to be caused by an ascending bacterial infection from the vagina which follows the development of Cystic Endometrial Hyperplasia (CEH). CEH occurs following repeated exposure to progesterone. Treatment of choice for pyometra is ovariohysterectomy, though medical management may be pursued in the short term for cases meeting specific criteria. Pyometra can be prevented by performing ovariohysterectomy in females not intended for breeding.